texts: Using a differential force in the derivation of hydrostatic equilibrium in a star

texts:
I've been banging my head against this particular derivation of hydrostatic equilibrium in a star for the last few days, from Carroll and Ostlie's Introduction to Modern Astrophysics (2nd ed. p. 286):

The authors consider an infinitesimal cylinder of gas (dimensions $A, dr$) aligned on an r-axis pointing away from the center of the star. Equilibrium demands that acceleration be zero, so we get

$$F_{bottom}  + F_{top} + F_G = 0,$$

Where the first two terms are the forces, taken normal to the surface, due to pressure, and $F_G$ is the gravitational force on the cylinder. The top of the cylinder is away from the center.

Alternative derivations I have found from this point onward make perfect sense- substitute $-\rho A dr$ for the cylinder mass $dm$ in the $F_G$ term, substitute $P_{bottom}$ and $P_{top}$, the rest is straightforward (this is the derivation on Wikipedia).

However, Carroll and Ostlie take an approach I haven't been able to understand. At this point they define
$$F_{top} = -(F_{bottom} + dF_P)$$
where $dF_P$ -I'll write it $dF$ hereafter- is called the differential force caused by the change in pressure due to a change in $r$. Substitution into the first equation and a few other steps here ($dF = A dP$) lead to $dP/{dr} = -\rho g$ as needed.

I have had trouble with the following: $dF$ needs to be negative because otherwise $F_{top}$ is greater in magnitude than $F_{bottom}$ which makes no sense- if $|F_t| > |F_b| $ then pressure isn't acting against gravity, is it? Pressure itself would be pushing the cylinder down in that case. ($b$ is 'bottom', $t$ is 'top'.)

But if $dF$ is negative, then the way the (second) equation is written makes no sense. Wouldn't it make more sense to say $F_t = -F_b + dF$ ?

Partially motivated by this question and partially trying to approach the problem from a slightly different angle, I decided to define $dF = F_b + F_t$ as the net outward force due to the infinitesimal change in pressure between the top and bottom of the cylinder. Then, as $F_{net} = 0,$ we get $dF + F_G = 0$, but this leads to an incorrect equation, namely $$ dF + F_G = 0 \implies dP/dr = \rho g . $$

My main issue is that I do not understand why trying to define a positive $dF$ fails to produce the correct hydrostatic equilibrium equation. Furthermore, if we view $dF$ as an infinitesimal quantity, trying to ensure its sign is consistent with the directions of the forces (outward being positive), we once again get the wrong answer.

Can we only take $dF$ as an inward force? If so, why? If not, how can I establish a positive/outward differential force that will let me derive the correct hydrostatic equilibrium equation?

Edit:

I think I can illustrate my dilemma a bit more succinctly.

Considering the equation $F_t = -(F_b + dF)$, we can try out 2 different possibilities: $dF$ being positive or negative.

If $dF > 0$ then $F_t$ is bigger in magnitude than $F_b$, in which case I don't understand how the pressure gradient creates an outward force because $|F_t|>|F_b|$ implies the downward force is stronger.

If $dF < 0$ then $F_t = -F_b + -dF = -F_b + |dF|$ but as far as I can understand, this form cannot, like any of my attempts to define a positive $dF$, output the correct sign in the final equilibrium equation. So computing the signs (that is, saying $(-dF) = |dF|$) somehow makes the equation incorrect. (Additional edit: For me, this also begs the question, if $dF = A dP$ then is $dP$ negative, and what does that signify?)


texts: Calculating 2004 BL86 position using software

texts: Is it possible to view asteroid 2004 BL86 using Stellarium or the iOS Star Walk app?


texts: Is it possible to see a spacecraft orbiting the moon with a pair of binoculars?

texts: On this clear early morning over London, I was studying the moon with a pair of binoculars and noticed a tiny dot of light that appeared to trace a slow orbit around the moon. I reckon it moved roughly 30 degrees in as many minutes. I'm wondering whether I was seeing a star or if it's remotely possible that I was seeing sunlight reflecting off an orbiting craft? Although the sky was clear it certainly wasn't starry; it was about 100 minutes before sunrise.


texts: What is the orbital path of the newly discovered star-less planet PSO J318.5-22?

texts:
Recent results from Institute for Astronomy at the University of Hawaii at Manoa claim that there is a Jupiter-sized gas giant planet that is independent of a star about 80 light-years from Earth.

The press release refers to this planet as "free-floating" but I assume that it must be in some kind of predictable orbital path.

What is the orbital path of this newly discovered planet?


texts: Why do some pulsar planets have designations with a capital letter at the end?

texts: The pulsar PSR B1257+12 has three planets. Most triple-planet system have designation foo b, foo c, and foo d. However, I noticed that the three planets of this pulsar (PSR B1257+12) have a designation of A, B, and C as well as b, c, and d. Why did this naming convention deviate from the standard?


texts: If an Asteroid was to strike the Earth, would it affect the Earth's rotation?

texts: If an Asteroid was to strike the Earth, would it affect noticeably the Earth's rotation, and if so, how large would this Asteroid have to be?


texts: Are the orbits of all triple star systems at least technically unstable?

texts:
Background:

In the Circular Restricted Three Body Problem (CR3BP, CRTBP) some halo orbits are mathematically stable. That means that the orbit of the third body is closed, periodic, and stable against small perturbations as long as the two primary bodies are in circular orbits around their common center of mass. See answers to Are some Halo Orbits actually Stable?

Question:

But for three massive bodies such as stars, if they are interacting only with each other and only gravitationally, are all orbits still at least technically unstable?

Yes some systems may last longer than the lifetime of the individual stars, or even the age of the universe, but strictly mathematically are there some stable configurations or are they all mathematically unstable, i.e. one member can eventually be ejected like this?


texts:
I have a question where I have to calculate the orbital inclination of a planet, so that it isn't a exoplanet but a stellar component. The mass of the star it is orbiting is 1.1 $M_☉$. Does the exoplanet has a special mass if it is a stellar component (for example 0.5 $M_☉$) or is there another relation?

The question was made out of a few sub-questions. It is about 51 Peg b. In the first question, I had to calculate the half long ax (=0.05284 AU), the eccentricity (=0.013) and the $mass*sin(i)$ (=0.500 $M_J$)
In the second question, I had to calculate the inclination at which the mass of the planet is 4 $M_J$ (=7.18°) and the inclination at which it wasn't a planet, but a stellar component.


texts:
So far I got to understand the expansion of space is not to be understood as stars drifting further apart through space. There's something more fundamental - e.g. you can't simply measure speed of it, nor tell where the center of the universe (the midpoint of expansion) is. Well, some smart people got me from knowing the answer that is "Simple, obvious, neat and wrong"  to "I have no clue what it is".

Is there a resource/method that would allow a layman to understand how to think about distortions of space like its expansion; to visualize and understand the concept correctly and without erroneous simplifications; understand it before trying to delve into underlying details of physics?


texts: How is it possible that we haven't discovered anything in the Oort cloud yet?

texts:
The Oort cloud is a hypothetical cloud of small icy bodies surrounding the Sun at more than 1000 AU. It is thought to be a vast reservoir of comets that occasionally get disrupted, sending comets towards the inner solar system.
I would guess that there are stellar occultations where a body from the Oort cloud passes in front of a background star, obscuring its light. This makes it surprising to me that we haven't detected any objects in the Oort cloud. Perhaps the sensitivity of our instruments is not high enough? Perhaps that in spite of the large number of objects that should exist in the Oort cloud, these occultations are actually infrequent, and you would have to wait tens of years before seeing one? Perhaps nobody has combed the data to find such signals?
Would any of our existing telescopes have been able to pick up the occultation if it was pointed in the right direction at the right time?


texts: Has the iron core of Mars really solidified?

texts:
In the Nova episode "Origins: Earth is Born", Neil DeGrasse Tyson states, "But Mars is just a fraction the size of the Earth, so it cooled more rapidly. And as it cooled, its molten iron core hardened. As a result, Mars stopped generating its magnetic shield." (February 28, 2004)

see: http://www.pbs.org/wgbh/nova/space/origins-series-overview.html#origins-earth-born

This comment from Tyson is consistent with what I have long believed. But I wonder.

The melting point of iron (Fe) is around 1538 °C.

see: https://en.wikipedia.org/wiki/Melting_points_of_the_elements_(data_page)

The the geothermal gradient for Mars is estimated to be around 6 °C per km.

see: What is the temperature 55 km beneath the surface of Mars?

If the estimate of 6 °C per km is the ballpark, then I would expect that iron is molten as a mere 256 km of depth. At the core of Mars (over 3000 km in depth) I would expect the temperatures to be far higher than 1538 °C.

It seems to me that it is impossible for iron to be in solid form at Mars' core.


texts: How did the moon's orbit become eccentric?

texts: The Moon's orbit is more eccentric, 0.0549, than most planets. I can understand that planets get eccentric by disturbing each other like under the late heavy bombardment. And likewise for multiple moon systems. But shouldn't the debris after the collision with Theia have been circulirized? Is its orbit being changed by the planets, at every opposition with Venus?


texts:
I am using DS9 to calculate the rotational velocity of a galaxy. I have 21cm cube and  I used DS9 to plot this graph

I do not understand what is FELO-HEL! and I want to convert the units from FELO-HEL to Km/s?


texts: Do all moons orbit their planets on the ecliptic? Do they all follow this same process as the moon that orbits Earth, or is their some random distribution with some even having extreme polar orbits.


texts: Does Jupiter help to protect the Earth against comets/asteroids impacts?

texts:
Does Jupiter help to protect the Earth against comets or asteroids impacts?

  Some astronomers believe that one reason Earth is habitable is that the gravity of Jupiter does help protect us from some comets. Long-period comets, in particular, enter the solar system from its outer reaches. Jupiter’s gravity is thought to sling most of these fast-moving ice balls out of the solar system before they can get close to Earth. So long-period comets are thought to strike Earth only on very long timescales of millions or tens of millions of years. Without Jupiter nearby, long-period comets would collide with our planet much more frequently.

From: http://earthsky.org/space/is-it-true-that-jupiter-protects-earth

Intuitively speaking, it would seem to me that while any massive body in the Solar System would deflect the trajectories of some comets/asteroids that would otherwise pose a threat to Earth, it also must be true that some few ones that were not in a collision course would then be directed to us after having their trajectories deflected. Why isn't that the case?


texts: What are the lines in this Solar Cycle graph?

texts:
What do the lines in this NASA graphic represent?

Why does Solar cycle 22 have a peak of nearly 300 (assuming that peak represents the maximum observations at the same time) and this link says the peak is 212)?


texts: What physical features determine if a planet is a major, minor or dwarf planet?

texts:
Like many, when I was growing up, we always were taught, hence always learned that there were 9 planets.  However, recently, decisions were made and all of a sudden there were 8 major planets and a series of dwarf and minor planets.

What physical features delineate a body being referred to as a major planet as opposed to dwarf planet or minor planet?


texts: What celestial mechanics parameters define a lunar occultation of Venus visible from Earth?

texts:
As a carry on from the following thread
Are lunar occultations visible to the naked eye?
Is it correct that the following parameters must hold true if we use Moon occult Venus as an example

[LongMoon = LongVenus = RAMoon = RAVenus] AND [DECMoon = DECVenus]
Moon and Venus must both be visible, i.e. not too close to the Sun

I expect both of these events will need to occur above the horizon (ascendant) for the location right?
Am I missing anything? Are there agreed parameters for degrees above the horizon for visibility?
Like eclipses, such events wont be observed everywhere on earth. Is there an orb convention that captures when occultations can be observed on earth? Is there a way to identify where on earth the total occultation can be observed?
Thank you for any guidance with this.


texts:
If the universe has formed & originated by a Big Bang Explosion, then there must be empty space left in the center of the explosion site, as all the matter is travelling at tremendous speeds away from the center, and there must be more matter, stars, galaxies and dust, etc near the present periphery or circumference or horizon of the present universe. As that big explosion has taken place about 13.7 billion years back, then the outer boundaries of our universe are 13.7 billion light years away from the centre of the explosion of Big Bang.

Have our astronomers discovered hollowness or emptiness anywhere in the centre of the universe or not?


texts: What does "located in the Hubble flow" mean?

texts:
I do not understand that much about the term Hubble flow.
For cosmological studies with type Ia supernova, I often see the sentence like "We used 100 SNe Ia in the Hubble flow". What does the difference between "located in the Hubble flow" and "not in the Hubble flow"? Thank you.


texts: Why can we see the Pillars of Creation, but not Proxima B?

texts:
I was wondering how is it that we can see the Pillars of Creation (I know they're no longer there too) which are labeled as 6,500 light years away from earth and yet Proxima B is labeled at just 4.2 light years away and yet we cannot see it clearly and it was very difficult to find?

I understand that the Pillars of Creation are giant gas bubbles with different molecules that make it so visible and Proxima B is a relatively tiny planet that doesn't emit light but I cannot fathom how much greater the distance is from Proxima B and yet it is clearly much more visible. Shouldn't our current technology be able to view 'close up' images of Proxima B?

What am I missing here?


texts:
On 2012 in the New Horizons' The PI's Perspective Alan Stern wrote The Kuiper Belt at 20: Paradigm Changes in Our Knowledge of the Solar System (more also archived) which includes:

Most of the known KBOs are just 100 to 300 kilometers across, about one-tenth of Pluto’s diameter. But some are smaller than 100 kilometers across, and some are larger than 300 kilometers across. In fact, there is great diversity among KBOs:

Some are red and some are gray;
The surfaces of some are covered in water ice, but others (like Pluto) have exotic volatile ices like methane and nitrogen;
Many have moons, though none with more known moons than Pluto;
Some are highly reflective (like Pluto), others have much darker surfaces;
Some have much lower densities than Pluto, meaning they are primarily made of ice. Pluto’s density is so high that we know its interior is about 70% rock in its interior; a few known KBOs are more dense than Pluto, and even rockier!

Question: How many Kuiper Belt objects have moons? How do we know this? What observational techniques have revealed moons of KBOs?


texts: What will happen when landing on Jupiter?

texts:
Jupiter is a gas giant, so landing on it will not be like landing on Earth, our Moon or Mars etc., as it does not have a solid surface like these.

If we have a hypothetical spaceship or probe landing on Jupiter, and it can withstand the enormous pressure, what or how will be the sequence of events?

I already read this:

  A major problem in sending space probes to Jupiter is that the planet
  has no solid surface on which to land, as there is a smooth transition
  between the planet's atmosphere and its fluid interior. Any probes
  descending into the atmosphere are eventually crushed by the immense
  pressures within Jupiter.

What I do not understand:

Is this enormous or immense pressure of atmosphere or Jupiter's surface?
While descending, will we know that we have transitioned from the atmosphere to the surface?
Why does animated footage (real or conceptual) of Jupiter appear like a high viscous liquid (e.g. lava) rotating in an opposite direction, rather that gas(es)?
Does Jupiter have an inner solid core?

I've already gone through these:

Is Jupiter made entirely out of gas?
Could we fly/drive through Jupiter?
If Jupiter is a gas-giant then why don't its features change?


texts: Why doesn't the fusion process of the sun speed up?

texts:
Am I correct in saying that the fusion process of the sun is constant, i.e. X amount of fusion happens per day, more or less?
Why does this not speed up, i.e. one fusion event creates energy for two fusion events, etc.?
Does every collision of atom cause a fusion event, or is the probability small for a fusion event to happen thus it's not a runaway reaction?
I have heard that the probability for fusion event to happen is only 1 in 1012 for every collision.


texts: Or in other words: which moons do have ionospheres but not surface-bound ones? The space beneath an ionosphere is called a neutrosphere. I only know of Titan. I know that the Galilean moons and Triton also have ionospheres but do all of them have surface-bound ones or do they have neutrospheres too?


texts: Both the laser collimator and cheshire eyepiece are tools used in astronomical telescope collimation. What are the main advantages/disadvantages of each? Which one might be best suited to start with for telescope newbies?


texts: Earth and moon gravitational ratios and proportionalities

texts:
This question is for an astronomy class of mine. I don't believe the question is particularly difficult, but rather I'm having trouble understanding what it wants me to do. We haven't covered ratios and proportionalities and as a result I'm very lost. Here's the question:

The gravitational acceleration due to a planet or moon tells you how “heavy” you feel when you stand on the planet or moon.  Gravitational acceleration is proportional to the mass and radius of the planet or moon in the following way:
$g\propto \frac{M}{R^2}$

Start by writing two proportionalities, one for the Earth, one for the Moon. Use the following variables:

$M_{E}$ : mass of Earth
$M_{M}$ : mass of Moon
$R_{E}$ : radius of Earth
$R_{M}$ : radius of Moon
$g_{E}$ : gravitational acceleration of the Earth
$g_{M}$ : gravitational acceleration of the Moon

Following the example under the ratio section, divide the two proportionalities to form a ratio, i.e., the ratio between the gravitational acceleration of Earth and the gravitational acceleration of the Moon. In other words, find the following in equation form:
$$\frac{g_{E}}{g_{M}}$$

This isn't from a lack of effort. I'm just not entirely sure how to write or divide a proportionality.


texts: Identification of Lunar mare (maria) from a taken picture

texts:
I'm in Brisbane right now. I took this picture 15 minutes ago from my balcony.

Could you help me with identification of marked Lunar maria? I cannot compare any of these to this map.

I think 1 is Mare Serenitatis and 2 is Mare Tranquillitatis. Am I right?


texts: Why do we need to build telescopes that can view in the infrared? What exactly do we need to see in the infrared region? I know this seems like a really broad question but I'm just curious about it.


texts: How can I contribute to the scientific community using my telescope?

texts: I've been using my Meade 90 ETX for a few years trying to become a better astronomer and am familiar enough with the basics to navigate my way around. I was wondering in what way an amateur astronomer such as myself can contribute to the scientific community. I would like to try and do more than just stargaze and take pictures.


texts: Did Edmund Scientific 4¼ inch Newtonians have 90° prisms as secondary mirrors in the 1970's?

texts:
I mentioned my old telescope's secondary mirror in this answer (stiffness) but I'm not certain I am remembering correctly. My first telescope was a Edmund Scientific 4¼ inch Newtonian reflector. This was in the 1970's. I seem to remember that the secondary mirror was a right angle prism rather than an elliptical front-surface mirror.

Am I dreaming or mis-remembering, or did Edmund use prisms on at least some 4¼ inch Newtonians?

Either way, any discussion on the tradeoffs between front-surface elliptical mirrors and 90° prism secondaries would be appreciated.

Source: John M. Pierce's HobbyGraph Articles HobbyGraph #15; Diagonals and Diagonal Supports.


texts: How to algorithmically distinguish between different types of nebulae?

texts:
I remember that there are five types of nebulae:

Planetary nebulae have not much to do with planets, but are emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
H II regions are emission nebulae in which interstellar atomic hydrogen is ionized.
Reflection nebulae are "clouds of interstellar dust which might reflect the light of a nearby star(s)".
Dark nebulae are interstellar clouds that is so dense that it obscures the visible wavelengths of light from objects behind it.
Supernova remnants are just that - the leftovers of a supernova

As Donald Knuth once said

Science is what we understand well enough to explain to a computer. Art is everything else we do.

I fully understand the definitions of the different types of nebula, but I struggle to write a computer-readible definition for each of nebulae types. What I mean: Given a deep-sky image with pre-identified nebulae zones, I do not know how a nebula type identification algorithm would look like. Do different types of spectra uniquely define each nebula type? That would work for the emission nebulae at least, I guess.
References
The answer is possibly given by Steven R. Coe's article What Are All These Different Types of Nebulae, and What Details Can I See in Them with My Telescope? which hidden behind a paywall and not accessible for me.


texts: Current topics on Radio Astronomy and looking for advice

texts:
Background and Question: I'm an undergraduate student interested on Radio Astronomy. I consider myself enthusiastic about it and I project myself following this "path" so I'd like, apologizing myself if this is a naive question, to ask you, more experienced astronomers, what are the current topics being part of research in Radio Astronomy.
My thoughts:

I know Cosmology and CMB Measurements are something that will keep its route for a long time as huge projects continue being developed.
Also I'd say Radio-loud Galaxies are something that Radio Astronomers are putting lots of work into.
Interferometric techniques, data reduction algorithms and image synthesis.
Astro-Chemistry and the study of spectral lines using data cubes such as provided by ALMA.

More questions:

Am I wrong with any of the topics listed? What else is being done now?
How do you see this in the next 5 - 10 - 20 years ?
Considering I want to work on my thesis project this year, what would you recommend (or to other undergrad. students) ? What would be a "good route" to follow after graduating? (my grades to this date are at ~ 80%).


texts:
Is gravity relative to volume, or size of an object?

Since a black hole is a massive star that collapses on its weight, how comes the same sun's mass, when it becomes a black hole, provides gravity that disallows even light to come out of the black hole? Before it was a black hole, the sun had a similar weight to the black hole, since no mass was lost or gained (and particles, including photons, come out of the star). Once it collapses under its weight, does the gravity become more in relation to the size of the object in question (reduction from the huge surface area of a massive star, to a compact/compressed, low surface area black hole), now that the same non-increased or subtracted gravity is bound to a much smaller surface?

Or in other words, what is it that makes everything bound to a black hole, but nothing of the same happens to a sun of similar mass, (and therefore with the same gravitational pull).


texts: Are exoplanets at dwarf stars less likely to have super-rotating atmospheres or asynchronous tidal locking?

texts:
Dwarf stars have terrestrial sized planets orbiting in habitable zones very close to them. These exoplanets are often said to be tidally locked to their star, like the Moon is to Earth, and that they thus have one hot hemisphere and one cold.

But in the Solar system there's only one tidally locked planet, and that is Mercury which is asynchronously locked in a 2:3 relation. It does rotate relative to the sun anyway. And only one known planet which practically doesn't rotate at all, Venus. But Venus has a super-rotating atmosphere that distributes heat from insolation all around anyway.

Are there reasons to believe that asynchronous tidal locking and super-rotating atmospheres are less common for planets with a dwarf stars than in a planetary systems with a Solar like star?


texts:
Comments below this answer to Why does the Solar Wind consist of charged particles? have led me to ask the following:

Question: Does plasma recombination in the solar wind happen to any significant or measurable degree?


texts:
According to the NASA JPL web-report, Gravity Gradients Frame Oceanus Procellarum, a rift-like ring structure surrounding the Moon's Oceanus Procellarum has been detected as a gravity anomaly by the GRAIL scientific mission (as can be as the dark blue features on the image below).

Image credit: NASA/Colorado School of Mines/MIT/GSFC/Scientific Visualization Studio

What mechanism could have formed the Moon's Oceanus Procellarum rift-like gravity anomalies?


texts: Which lens and magnification is best to see planets?

texts: I just bought a Celestron Travel Scope 70 and it comes with a 20mm and a 10mm eyepiece. I get a good view of the Moon but I am not able to see Jupiter, Saturn or Mars as expected. I read about the Barlow lens but I am not sure how much magnification is required to view these planets. So based on my telescope, which lens should I go for? And should I also get Red/Blue filters to view them?


texts: Why is $H_\delta$ prominent in type A stars?

texts: I understand factually that H$\delta$ lines are most prominent in type A stars and less so in more extreme types of stars on the H-R diagram. However I was wondering the reason for why they are not prominent in more extreme type stars such as type O or type F. For type F stars I believe (this may not be correct) none of the hydrogen is in the 4th excited state (or very few are) so there aren't many H$\delta$ balmer lines. However I am very unsure of why the EW H$\delta$ is low in type O stars. Can someone please explain this, Thanks!


texts: Is there any positive N on which it's agreed among astronomers that 10^-N is accepted as 0 (zero)?

texts:
Is there any positive N on which it's agreed among astronomers that 10^-N is accepted as, or treated as 0 (zero)?

I think one of the sciences that most precision is required is astrophysics.

I'm aware of my hardware capacity to lowest floating number, but that's not what I'm asking about.


texts: What spectral type of star has an absolute magnitude of exactly 0?

texts: We know that Vega is the star that serves as the zero point for the UBV color scale, and has an apparent magnitude of nearly zero (+0.02). But its absolute magnitude is +0.58, making it rather far from $M=0$. So what spectral type would have an absolute magnitude of $0 \pm0.02$, and are there any stars that satisfy this?


texts: Can a natural satellite exist in a geostationary orbit?

texts:
While browsing through Physics SE, I noticed a question about satellites in geostationary orbit (unrelated to the one I'm asking here), and for a moment I interpreted it as referring to natural satellites (e.g. a moon). So I wondered: Could a natural satellite exist in geostationary orbit?

Then I stopped and thought. For large gas giants, such as Jupiter, having moons too close to the planet can be fatal (for the moon). If it ventures inside the planet's Roche limit, it's toast. But there is good news: the Roche limit depends on both the masses and densities of the primary body and the satellite. So perhaps this reason is non-applicable, as a high-mass natural satellite might be able to survive. So the question changes:

Could a sufficiently high-mass, high-density natural satellite occupy geostationary orbit over its primary body?


texts: How can I get the exact time of each zodiac every year?

texts:
I want to make a calculator for Chinese New Year calendar. There is a rule that a leap year happens if the eclipse happens before each zongqi (equivalent to the middle of zodiac sign, ie. 15º, 45º, 75º, 115º... to the sun).

Where is a good reference to know that the exact time of the earth entering each zodiac sign every year?


texts: What will be the eventual result of the Milky Ways future mergers?

texts:
I am wondering, after the Milky Way merges with Andromeda and other near by dwarf galaxies and globular clusters, will it then merge with galaxies in the Virgo cluster and even Laniakea? And if so will the eventual result be one super giant galaxy or smaller but still large galaxies orbiting each other whilst the rest of the universe has expanded out of sight?

Edit: during my research I have had contradicting information on what will happen some say within each cluster, galaxies will all merge to one giant galaxy, whilst others disagree?


texts: How did the orbits of Phobos and Deimos circularize?

texts: The satellites of Mars might be captured asteroids. But Phobos has an eccentricity of $0.015$, and Deimos of $3\times10^{-4}$. If they were captured, they likely were captured with rather eccentric orbits. How did their orbits circularize?


texts: Why don't we see "the milky way" in both directions?

texts:
We're (basically) in the middle of an arm of our galaxy. That is to say, we're sitting in the middle of a dense disc of stars.
It would seem to me that. You should see:
the thick line of the milky way all around you, i.e. on that plane, in all four directions.
(Additionally - sure - in the particular direction of the galaxy center, you would additionally see the huge central bulge.)
However: this does not seem to be the case: when you look at the milky way from the vicinity of our solar system, you basically see it "in one direction".
What am I misunderstanding?  How come the sky object "the milky way" is well-known as only a lump/strip in one direction, rather than, a lump/strip that goes right around us?
--
Put it this way...
Quite simply does anyone have any photography of the anti-galactic point?  (Near "Auriga" right?)  Does it show any "milky way band" going through it?
If not why not? Looking outwards, we are still looking through ~30k lightyears of the dense disk we are sitting in.


texts: Are the planets Mercury, Saturn, Uranus and Neptune losing mass?

texts: My understanding is that Venus, Earth, Mars, and Jupiter are all losing mass due to their gravity, temperature, and the solar wind.  But what about Mercury, Saturn, Uranus, and Neptune?  Mercury has a magnetic field and so typically would repeal the worst of the solar wind and no atmosphere to lose, only an exosphere.  I do not know the calculations to determine if Saturn, Uranus, and Neptune are losing mass though.


texts: Does red shift need to compensate for space expansion

texts: Distant galaxies are moving as space expands, not moving through space.  So a photon coming to us from them has to come back through distorted space.  So is not spacetime distorted, so the photon has to go through an area where time slows down.  So would this cause red shift.  So maybe we have a simplistic view of red shift and maybe those galaxies are not going as fast as we think


texts: Eclipses of Jupiter's Moon during Retrograde motion

texts:
When Jupiter is undergoing retrograde motion as seen from the Earth, would you expect the eclipses of Jupiter’s moons to occur several minutes early, several minutes late, or neither?
Can someone please explain?

I am a novice in astronomy, and I read a couple of articles on the retrograde motion. I understand it is an illusion. But how would I expect the eclipses of Jupiter's moon?


texts: How frequently does a comet drastically change its orbit in the Solar System (like 67P did in 1959)?

texts:
Comet 67P/Churyumov–Gerasimenko flew by Jupiter at only 0.05 AU distance in 1959. This encounter lowered its perihelion from 2.7 to 1.3 AU (almost becoming an Earth crossing comet). Since it happens to be the target of the Rosetta mission (and was picked only because of a launch delay), I wonder if it is as common as it seems that a 4+ billion year old comet shifted its orbit drastically less than 60 years ago?

How often does Jupiter do something like this?

How many known comets are known to have had their orbits substantially changed by a single flyby?


texts:
A recent question at Space.se has generated a few comments that are interesting.

But the answer to one particular comment to might address a lot of misconceptions.

  I imagine the actual halfway point is close to the sun Source

As the two planets orbit the sun at different speeds, I assume it must be a significant amount.

How long is the Sun closer to Earth then Mars is?


texts: If an earthquake happens on Mars, is it still an earthquake?

texts: Or are seismic phenomenon named differently when they happen on other celestial bodies? If so, what are they called?


texts: I'm making a new website for the folks at MIRA.org, and in a bit of curiosity I found myself wondering if there are observatories that are for-profit organizations. What would they even do?


texts: What is the reason for the dispute over the variation of core mass with heavy metal abundance?

texts:
In Metal-Poor Stars IV: The Evolution of Red Giants, Rood writes

  The differences in the results of these papers are large enough to introduce appreciable uncertainties into the study of the horizontal branch. For instance, there is still disagreement even as to the sign of the variation of core mass with heavy metal abundance ($dM_c/dZ$).

What is the cause of the dispute?


texts: Could there be Earth-like planets in binary or trinary systems?

texts:
Could Earth-like (suitable for Human habitation) planets exist in systems with 2 or 3 suns, have any been found?

With more than a single star, isn't it likely that the habitation zone would be too irregular, gravitational forces be irregular/strong and numerous other issues?


texts: When is optical refraction important in astronomy?

texts:
What are commonly important astrophysical systems/models, where optical refraction is important or necessary to account for?

I would kindly ask you not to consider refraction in Earth's atmosphere or inside instrumentation in this question.

Comment: Gravitational lensing is different from optical refraction even though it affects light paths. I would like to ask the authors to avoid mentioning it in the following.


texts: Can someone explain me this diagram from an article?

texts: How should I understand this 59 degree mentioned below, what does it mean? I could not get it. The photo is a diagram from an article dedicated to the invention of the ecliptic. The photo is from my tablet (the title could be seen)


texts: Can Hubble red shift be interpreted as time dilation?

texts:
Can we interpret the de Sitter universe as a spherical cosmic horizon null surface of finite radius, centered at Earth, and containing the Hubble volume of space where time is dilated and radial dimensions contract closer to the edge in such a way that objects closer to the edge do not recognize that they are radially contracted?

Everything is attracted to the edge, but the total radius remains more or less constant and emits de Sitter radiation at finite temperature.

P.S. I know real universe is not de Sitter but it is usually asserted that it approaches de Sitter state asymptotically.


texts: can i use heliocentric velocity as a rotation speed?

texts: In the research of the galaxies, does heliocentric velocity involves space's expanding velocity? and can i use heliocentric velocity by galaxy's rotation speed?


texts: What are these dark spots on Mars' north pole?

texts:
I was exploring Mars in Google maps and I found these dark spots, like tree shadows on the north pole. I don't have any idea what they might be.

View this in Google maps.


texts: Where can I find a dataset containing the interstellar reddening and extinction values for all globular clusters, preferably in the HST/WFC3 system.


texts:
By using only the electromagnetic signature of a star, could a star be distinguished with reliable accuracy from any other star?
To elaborate a little, say we have a collection of about 200,000 stars. By using EM spectrum alone (and any means of analysis, such as spectral analysis), and by tagging each star with its corresponding EM signature, could there be similarities too close to fail a distinction between any two or more stars?

Now, of course, I know that such a thing is possible. I'm not asking whether it is possible. I just want to know the odds of such a lack of distinction, considering all the factors of stellar evolution. It's either in the ballpark or not.


texts: Can the interferometer called "Gravity" measure "a few centimeters on the Moon"?

texts:
Phys.org's Very Large Telescope sees star dance around supermassive black hole, proves Einstein right links to several ESO videos, including Interview with Reinhard Genzel (in English). After 08:51 in the video professor Genzel says:

So we take four 8 meter class telescopes of the European Southern Observatory, the Very Large Telescope in Chile, and combined it optically to an equivalent 130 meter telescope, and the bigger the telescope, the better the resolution.
So with this interferometer, called Gravity, we can measure basically a few centimeters on the Moon if you like, and that gives us the precision to really you know nail the motion of a star around a black hole.

To the question Is seeing the apollo moon Landers via earth telescope that hard? I recently wrote Yes, it still is. and then asked Will the Magdalena Ridge Optical Interferometer be able to image extended objects like the surface of the Moon?, but if Gravity can resolve a few centimeters on the Moon then it should be capable of photographing all of the Moon landing sites.
Question: Did professor Genzel mis-speak, perhaps meaning that the resolution is a few centimeters at the distance of the International Space Station, or does Gravity at least theoretically have a resolution of a few centimeters at the distance of the Moon?
I know that three orders of magnitude is fairly small for astrophysicists, but since the reference is The Moon and there's stuff on it that we'd like to see, in this case enquiring minds want to know!
FYI there is also Interview with Reinhard Genzel (in German); it might be interesting to compare.


texts: Are there only a set number of atoms in the whole universe?

texts:
As per the title, are there only a set number of atoms in the universe? Or does the universe make more atoms, and if so, how does it work?

I'm a high school student that just had a question during science class, I didn't ask my teacher, since he has more knowledge in biology than anything.


texts: How to calculate the altitude of the Moon?

texts: With the known latitude coordinate of the observing position, how to find the altitude of the moon when it is high, i.e when it crosses the local meridian?


texts: What is the percent flattening in Haumea?

texts:
I've recently noticed that in Wikipedia articles such as Pluto (Link), there's this number that says how much flattened it is (Definition Here). In Pluto, it's >1%.

Haumea, which is a very flattened dwarf planet, actually doesn't have this information on Wikipedia, and I haven't been able to find it online.

It would be great if someone could find this info (and it's source as well), because I can't seem to find it anywhere.

Thanks in advance!


texts: Have all Wolf-Rayet stars evolved off the main-sequence?

texts:
From what I've read from various sources, Wolf-Rayet stars are massive, evolved stars that typically are the hottest, most luminous stars known. They experience high mass loss, taking their mass from $\sim 100 M_\odot$ to only $\sim 20 M_\odot$. However, Wolf-Rayet stars like R136a1, 2, and 3 are still supermassive, at around $150-220 M_\odot$, and are about only 1 million years old.
Does this mean that these stars already have burned their hydrogen and have evolved off the main sequence, or are these stars still on the main sequence but are classified as WN5h as a result of its high surface temperature and luminosity?


texts: Does the ratio of daylight change from year to year?

texts:
Over a year, the ratio of daylight changes, e.g. in Montreal there is more daylight in the summer than in the winter.

I want to know if, given a bad data sample, e.g. five years, one might notice a trend of increasing or decreasing daylight ratios.

Here's why I ask. I'm doing research on the Dead Sea scrolls, and some of the fragments contain ratios that a scholar has argued represent daylight. What I'd like to know is a trend that the community of Qumran might have noticed in their dataset. They had a theory that darkness was increasing. I doubt their data was exhaustive, but I wonder if around Jerusalem there might have been a period of decreasing daylight ratios over a period of a few years roughly around 100 B.C.E.


texts: If put enough weight on a particular point on Earth's surface disturbing the balance between hemispheres, is it possible that the Earth's spin could change like an unbalanced spinning top?


texts: Would it be possible that Mercury had a natural satellite but the gravitational pull from the Sun became so overwhelming from it growing over the years that it just sucked in Mercury's moon?


texts: What values can a color index variable get?

texts:
I am a statistician with love and passion for astronomy. I try to explain some outliers I found in the values of quasars - color indexes. I found four (4) values close to twenty (20). Is that scientifically possible or there is an error in the CSV file? What values can a color index get?

Ty in advance.


texts: Why isn't the star that created the black hole a black hole?

texts:
If the mass of a black hole is creating so much gravity that light cannot escape, why isn't the mass of the star that created the black hole (before it went supernova) trapping light as well?

By all accounts, that pre-supernova star should have boatloads more mass than the black hole post supernova, right?  Doesn't the star lose most of it's mass when going supernova?


texts:
I've got a Celestron Astromaster 114 telescope. I live in Los Angeles and would like to get a look at 46P/Wirtanen. Can anyone tell me when this comet is going to brightest and how best to locate it in the sky? It's my (perhaps incorrect) understanding that it is not especially bright. Any tips/hints/information would be much appreciated.

Source


texts: I was just looking up something about Uranus on Google, and read a line I had read many times before - "The strong winds of Uranus...". So, short and easy - "What causes the strong winds on Uranus?".


texts: What is driving the expansion of the Universe?

texts:
Is it just the momentum from the Big Bang? If yes, how can it account for the acceleration of the expansion?
Why isn't the collective gravitational force slowing it down?
What part does dark energy play in the expansion?


texts: How does one measure velocities of far-off, bright objects

texts:
As the title already says, I want to know how one measures velocities of far-off, bright objects, e.g. when the mean parallax drift isn't measurable with current apparatus (this means when there is only a "non-moving" picture of that object available)

I know that one can measure the redshift of spectral lines and correct them for gravitational redshifting if the distance is already known.
But then the relation between source-frequency $f_S$ and observed frequency $f_O$ is
$$ f_O = f_S \frac{\sqrt{1-\frac{v^2}{c^2}}}{1+\frac{v}{c}\cos(\alpha)}$$,
where $\alpha$ is the angle between line of sight and the velocity vector.

If one only has a "non-moving" picture of the object, there is no way to determine this angle $\alpha$ and therefore no way to get the velocity by means of redshift.

As an addition: At which distance does the mean parallax drift become unmeasurble with current apparatus?


texts: Equating critical energy density to matter density

texts:
I'm asked first to calculate the critical energy density $\rho_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$.

Then I'm asked to calculate various number densities of particles assuming the universe is made up of a single type. To do this, I'm told I need to set the universe's matter density equal to $\rho_{crit}$.

I'm not sure how to do this since they are in different units. My expression to find $\rho_{crit}$ had a $c^2$ in it, and if I take that out I would have units of matter density. Is that what I'm supposed to do?


texts: Distance to Sun at Perihilion and Aphelion at max and min Eccentricity

texts: Title says it all. I'm putting together a presentation on Milankovitch Cycles, and I would like to bring up the difference in distance to the sun (km) at aphelion and perihelion when the earths orbit is at maximum and minimum eccentricity. The numbers I've found are that the maximum eccentricity of Earth's orbit is 0.0679, and the minimum is 0.000055.


texts: Can Hawking Radiation ever be detected and does it exist?

texts: I'm researching black hole Hawking radiation for a contest, and so far I get the overall gist of how the process works. But as far as I've searched, there hasn't been any confirmed observational evidence that it exists, other than laboratory simulations/recreations. Although the theoretical side of it makes sense, how do we know it exists?


texts: Angle formed between a satellite, a ground station, and then another Satellite

texts:
I am trying to check my work on calculating the angle between two sats and a ground station observing those two sats. I have the az and elevation angles to each sat from the ground station. What would be the equation to get the angle Sat1-GS-Sat2?

I know that it uses the law of Cosines, but not sure if I was using the correct equation.  Here is what I am using:

Angle = acos((abs(sin("V_Sat_1_EL")*sin("V_Sat_2_EL")))+(cos("V_Sat_1_EL")*cos("V_Sat_2_EL")*cos(abs("V_Sat_2_AZ"-"V_Sat_1_AZ"))))

Thanks


texts: at approximately 5:45 a.m. Est,I was 20 "stars" moving w to e faster than an airliner. They were about 60 degrees off the southern horizon and took several seconds to cross the sky here in central-western pennsylvania. Satellite's? March 1, 2020.


texts: The # of Estimated Galaxies increased 10 fold. Where does that leave our # of stars estimate?

texts:
I have found oddly different answers through my multiple google searches.

I found one article that mentioned this did not alter the estimated number of stars in the universe. Link: http://www.universetoday.com/30305/how-many-galaxies-in-the-universe/

Yet other article gave the estimate of stars an increase. Link: http://gizmodo.com/we-were-very-wrong-about-the-number-of-galaxies-in-the-1787750693

There were other examples but they also had similar contradictory answers.
Thank you.


texts: Structure of the torus of the Active Galactic Nucleus

texts: How much do we know about the structure of the torus surrounding the Active Galactic Nucleus (AGN)? Is the shape defined or still under speculations? And are there different structures for this torus possible i.e., does it vary from galaxy to galaxy?


texts: Chemical reactions in white dwarf and carbon allotropes

texts:
White dwarfs consists mostly of carbon and oxygen. In my opinion, they are too hot to contain these elements in molecular form and hence chemical reactions does not happen (I think resulting CO2 will also decompose under such high temperature and pressure). I have two questions related to that:

1) After a few billion years, can the WD cool down enough to sustain chemical reactions which will result in production of CO2.

2) Given, there is very high pressure and temperature, after a few billion years what will be the dominant allotrope of carbon (Do we have, in future, diamonds in the sky ;)

I strongly doubt given the extreme density of white dwarfs( I think density of white dwarfs wont change over time), if any of the scenario is possible.


texts: Why do planets and satellites in the Solar system look so wildly different if they came from more or less the same matter?

texts:
First, the planets. We have Mercury, which is rocky, no atmosphere. But then we have Venus, which is completely different: thick atmosphere, very hot, geologically active. Then Earth - blue, full of water. Mars, the opposite: red like nothing else. Jupiter and Saturn are fairly similar. Then Uranus and Neptune, fairly similar but still differ in color between each other and also totally different in color than the two gas giants.

On the other hand: satellites. Let's analyze satellites of Jupiter and Saturn.

Ganymede and Callisto fairly similar, but then Europe, total opposite: completely icy. And then Io, again something completely different: strikingly yellow.

Saturn's moons: mostly rocky, but then, something completely different: Titan, with a thick atmosphere like no other satellite and oceans of liquid methane.

If during the formation of the Solar system there was a protoplanetary disk of matter, wouldn't it be pretty homogenous and therefore give rise to similarly looking planets? I understand that gas giants can't look the same as rocky planets, but why are there differences even between similarly sized rocky planets? Granted, there are wildly different temperatures throughout the Solar system, depending on the distance from the Sun, which probably explain some of the differences.

But then what I especially don't understand are the differences between the satellites. If say Jupiter had a disk of matter orbiting it, which eventually formed into satellites, wouldn't at least that "local" disk around a planet be fairly homogenous? But nevertheless it developed into wildly different satellites. For example, how did the "yellowy" thing get concentrated on Io, and not equally distributed on all the Jupiter's moons?


texts: Do the orbits of satellite galaxies show the influence of dark matter?

texts:
I understand how galaxy rotation curves show the influence of dark matter, or something else that would produce similar behaviour, such as Modified Newtonian Dynamics (MOND). Do the orbits of satellite galaxies show the same influence?

In MOND terminology, the internal gravity of the satellite galaxy is not relevant to this question. The question would be if the gravitational effect of the host galaxy on the satellite was Newtonian or MOND?


texts:
I need to find metal lines in the Lyman-alpha forest of quasar spectra, without the red side of lyman alpha emission line.
Can you guide me how to do this?


texts:
Straight from my 7 year old to you, exactly what it says on the cover:

  What is the hottest thing in the universe?

To make it Stack Exchange-friendly, I'll add the following caveats:

it should be bounded, as in an actual compact object, or class of objects, or part of an object
it should be observable
it should be an astronomical object, ie a Quark Gluon Plasma created by collisions at the Large Hadron Collider doesn't count.

Thanks,
Bruce


texts: Why does the Sun have different day lengths, but not the gas giants?

texts:
The Sun's rotation period varies from about 25 days at the equator to about 38 days at the poles. As I understand it, this is because the Sun is not solid, and because of the way centripetal force works, the equator must move faster than the poles.

Question: if this works, why do Jupiter/Saturn/Uranus/Neptune have well-defined days? Why don't the equators of these planets rotate faster than the poles as well? For example, Wikipedia's article on Jupiter gives the length of a Jovian day as 9h 55m 30s, which is so precise that it implies Jupiter does not have a rotational period which varies with latitude.


texts:
I often hear that nothing can escape a black hole because its "escape velocity" is greater than c. If that is accurate, what about the following? I know that the following has a lot of most likely impossible assumptions, but I'm trying to understand whether the escape velocity explanation makes sense.
Suppose you've got some sort of space ship that's inside the event horizon, and it hasn't been destroyed by the gravity or tidal forces. Also suppose that you've got a whole lot of reaction mass and a fantastic power supply.
Could this space ship, through applying enough constant force escape the event horizon?


texts: What is there in the intergalactic space?

texts: What bodies can be encountered way outside galaxies - far beyond the farthest edges of galaxies, in the deep space between them? Are there single, galaxy-less stars, giant clouds of gas more dense than that the lower-than-elsewhere content of free hydrogen in space, or any other phenomena that span the void between the galaxies?


texts:
This question poses the question of why the Earth is still hot after 4.5 billion years, and the primary reasons were:

  (1) heat from when the planet formed and accreted, which has not yet
  been lost; (2) frictional heating, caused by denser core material
  sinking to the center of the planet; and (3) heat from the decay of
  radioactive elements.

With that in mind, would it be possible for a planet of Earth's size and age to have a cold core, mantle, and crust?


texts: Our galaxy, the Milky Way, forms part of a galaxy group called The Local Group. What are the parameters that dictate what a galaxy group is?

texts: Are there different classifications or just the one? How does our galaxy group compare with others?


texts: Does lack of atmosphere on the moon make earth-shine more, or less, bright?

texts: The earth acts like a light-globe in the sky for half of the moon, the side that is facing the earth. In what way does lack of moon atmosphere influence earth-shine. Does it make earth-albedo brighter, or dimmer? Either way, do we know with how much? Put differently; would an atmosphere make it easier, or more difficult, to read a newspaper on the earth facing lunar surface, after sunset?


texts:
I keep hearing and reading statements that refer to the "surface of the Sun" (how hot the surface of the sun is) or the "surface of Jupiter" (when the Shoemaker comets hit Jupiter). I find this to be very confusing and unscientific (especially when stated by astronomers).
If the Sun and Jupiter are basically balls of gas they don't have a surface. It is like saying that the surface of the Earth is somewhere in the upper atmosphere.
Can anyone help me understand how a ball of gas can have a surface.


texts: What are the smallest star clusters affected by Galaxy Rotation Curve?

texts:
We all know that Milky Way exhibits Galaxy Rotation Curve which makes it rotate very differently that Newtonian mechanics would expect. But Milky Way is a pretty large galaxy, and the one we only see inside, so maybe it's not that good of an research object.

What are the smallest galaxies or star clusters whose rotation is clearly non-Newtonian? Are dwarf galaxies' rotation curves affected? Are globular clusters inside galaxies such as ours affected, or is their movement decisively Newtonian?


texts: What planets or exoplanets orbit the Sun’s elder twin HIP 102152?

texts: Has anyone done any research on the planets that orbit the HIP102152? Since that star is similar to ours and older, I postulate that it’s likely those planets are most worthwhile searching for advanced civilizations.


texts: Energy conservation in Interplanetary orbit transfer

texts: When a spacecraft flys past a planet eg. Jupiter , its heliocentric energy increases dramatically allowing it to go further away. How is this possible without using any more rocket fuel?


texts: Difference between first (focal length (fl.) 500 mm, ocular (o.) 10 mm) and second telescope (fl. 1000 mm, o. 20 mm)? (both 50 x)

texts:
Magnification is defined as $f_{objectiv}/f_{ocular}$.
If I had the first telescope with the focal length of 500 mm, ocular 10 mm and second telescope with the focal length of 1000 mm, ocular 20 mm, is there a difference between them? Both of them has 50x magnification.

I want to know if there is any difference in the quality of image, etc.

Thank you for all!


texts: Smallest body that can be observed from Earth?

texts:
What is the minimum size of an object that can be observed from Earth with our most powerful telescope?

I understand that the answer depends on how far the object is from Earth. For the sake of  discussions let us consider distances less the orbit of Neptune.


texts: The anomaly of spiral galaxies and the need for dark matter

texts: Please could some explain this to me - I understand that some galaxies are rotating with a speed that the stars on the outer reaches should not be held within the spiral given the observed matter and consequent gravitational field it forms. As a consequence of this, we are looking for dark matter that would compensate for this imbalance. But my question is this, how do we know that a black hole found at the centre of many galaxies is not providing the ‘extra’ gravitional curved space time required to hold the spiral galaxy together ?


texts: How to get from the earth to another planet/ the moon? What happens on the way?

texts:
I have a problem explaining for my students how to get from the earth to the moon? Is there someone who can give a good explanation?
I Think these are words that might be included. Escape velocity, gravity, force, free fall, weightless, earth orbit, lunar orbit.(How do the spaceship get into the lunars orbit how do the spaceship get from lunars orbit to the surface of the moon?)


texts:
I am a beginner.  Please let me know whether earth has retrograde motion

If it does not, let me know the reason


texts:
I've found this website (click here for the link), the link leads you to a problem about the velocity Andromeda is going towards the Milky way galaxy, and you can try three approaches. I like to challenge myself, so I click on the second one, about spectra. I already know what the Electromagnetic Spectrum is, but this one is a tad different. It tells you about the redshift equation, which I already know about, and I've used it a few times in examples I made for myself, but I am confused about the next page (to get there, click the button at the bottom that says "Use This Technique to Solve for M31's velocity"), and the next page is worded in a way that I can't really understand it, and using the clues I get from the Spectrum Graph, I can't figure out what the answer is. What is Relative intensity of light, and what does the Calcium have to do with it? Also, isn't their value for the speed of light inaccurate, as far as I know, it's $3\times 10^8$, not $3\times 10^5$. Even further on, they want you to calculate the radial velocity from the graph, which I have no idea how to do.
If an answer about this here is acceptable, please give me a feasibly worded explanation of all this madness, and thanks in advance!


texts: How can we focus radio telescopes on a star when the earth is spinning?

texts: Reading about the Star KIC 8462852, it has been said that the SETI project turned its radio telescopes towards the star to search for extra terrestrial radio signals as the star had strange fluctuations in light. How can we point from earth a radio telescope towards a star which is 1480 light years away whilst the earth is spinning at 1675 km/h and keep it focused or, in the case of a radio telescope, aligned in order to try and receive radio waves??


texts: Need help with telescope mount; stars have tails

texts:
If I take a shot longer than 20 seconds, the stars get a tail. I have a Celestron Advanced GT mount with starshoot autoguide scope. I use a cannon can on a 9" Celestron scope. I can't get a picture without having tails on my stars. It only looks good if I keep the pictures under 20 seconds.

EDIT :

I did the polar alignment. The mount has a polar alignment scope threw the center of it. It's also balanced, the mount has a brids eye level. I use the new PhD software with the starshoot autoguide.. There is no slop in the mount, the gears have almost no backlash.


texts:
Referring to this answer to What's the rationale behind the false colours in solar observation photographs? which includes the table from Wikipedia's Fraunhofer lines:
In the Table of wavelengths there is an e designated Hg line of 546.07 nm which DOES NOT appear in the picture of the Fraunhofer spectrum, any explanation? Also there are about a dozen lines in the spectrum which are not tabulated and not identified at all.


texts: Obtaining TESS input catalog (TIC) IDs for Gaia `source_ids`?

texts:
The TESS input catalog (TIC) "uses the GAIA DR2 catalog as a base" according to the catalog's website. The Gaia DR2 database provides source_ids for each observation of a target. I want to get the TIC number for a target based on the Gaia source_id. Is there an accepted way of doing this that I'm missing?

To be more specific, I can get a list of Cepheids from Gaia DR2 using their archive and the ADQL command select * from gaiadr2.vari_cepheid. The first result has the Gaia source_id as 4052922352453886976. How can I find the TIC number for this target star? Is the best way of doing this using the RA and DEC of the Gaia observation and finding the closest match in the TIC? Since the TIC is based on the Gaia DR2, I would assume there would be a direct corresponding target and finding the closest RA and DEC match would be unnecessary.


texts: Could a dwarf galaxy host a star at the center instead of a SMBH?

texts:
I don't think this would be possible, but I'm curious, and as what ever query I googled I only found posts about black holes indeed existing, what black holes are, that even the Milky Way has a black hole in its center or that We can't know if dwarf galaxies have non-SMBH at their center.

So sorry if it is a stupid question but I didn't want to go on with assuming something without knowing: So is it theoretically possible for a Galaxy to have a star at its gravitational center of mass?


texts: How much could Earth's orbit shift without causing an ecological catastrophe?

texts:
Seeing the news about A/2017 U1 made me wonder what would happen if a similar object, but with a mass close to Earth's, had a close call with Earth. Could it shift Earth's orbit enough to quickly and significantly change the conditions for life?

This answer sort of gets it it, but it's not clear how much of an effect the change in orbit would have.


texts: When and where are Pluto and Neptune closest to each other?

texts:
I found two sources sort of answering this question, but conclusions I made of them contradict each other.
https://www.reddit.com/r/askscience/comments/29jrvq/what_is_the_smallest_possible_distance_from_pluto/ - here is stated that Pluto and Neptune never come closer than two billions kilometres from each other.

Yet from picture in here (I am aware it might be not to scale) Can Pluto and Neptune collide anytime in future? it seems that Pluto and Neptune got a lot closer to each other shortly after 1999 than a half of Neptune orbit radius, judging by their speeds and positions in 1999.

So, how is it? How close can they get to each other and when is it going to happen next time?

Tried to use wikipedia, but while article about Pluto contains information at least about when it was in Aphelion, article about Neptune does not any such information (or I could not find it).


texts: How can there be anything "beyond" the CMB?

texts:
Two things we take, for this purpose, to be axiomatic:

The CMB is the oldest thing we can observe directly
The cosmological red shift tells us how "old" something we are observing is

Yet we also have a third axiom:

There are some parts of the universe we can never observe because they are receding away from us at a superluminary speed.

But this surely also implies that we can see beyond the CMB if we see anything which has a red shift indicating an expansion speed very close to $c$.

This suggests axiom 2 is incorrect - so what should it be instead?


texts: Is it possible that all dark matter is made of rogue planets (free-floating planet)? (and other stuff like asteroids or meteoroids)


texts: Finding the number of stars n light-years from the sun

texts:
The nearby stars seem homogenously spaced out enough to give a general "stellar density."
Does there exist some kind of mathematical expression that can determine the number of stars n light-years from the sun?
Or—
Is there a rough value for this supposed stellar density idea? (Ex: 0.3 stars per cubic light-year.)


texts: Clarification on Greenwich Sidereal Time (GST)

texts:
I know that LST = HA + RA.
And GST is LST + longitude(L).
If a star culminates, it's HA=0. Therefore, GST = RA + L.
Since RA and L are constants, GST must also be a constant. So, how can GST be different on different days of the year for the culmination of the same object for the same observer? Also if I'm given the L and RA, why am I given the GST in this question?


texts: Can a pulsar have an axial tilt close enough to $90^\circ$ to hit us with both beams for two pulses per rotation?


texts: Can you see the stars when it is trickling?

texts: I am writing a book with the following scene in one chapter: It is trickling and my protagonists are outside looking into the night sky, looking to the stars. Reviewing the scene I am wondering: Is this possible? Can you see the stars when it is trickling, or even when it rains? Are there images showing this?


texts: Why are most lunar maria on the visible side?

texts: Is there any reason why the Earth side of the Moon has enormous plains and mountains, while the far side contains just few small ones inside largest craters?


texts: In several equations of orbital elements (such as the determination of true anomaly from mean anomaly), the terms 1-e and 1+e appear. These are the ratios of the orbital periapsis and apoapsis to the semi-major axis, but do the ratios themselves have names?


texts: Determining Horizontal Coordinate System data (azimuth, altitude) for geographic locations

texts:
I would like to determine, specifically, azimuth and elevation data for any given location on the surface of the earth relative to another location on the surface of the earth (for my purposes, the latter will be London, UK).

Data and APIs are readily available for certain celestial objects, but I haven't had any luck finding data as required.

Can anyone enlighten me as to a method for calculating such data?

Many thanks.


texts: Io is the only body in the Solar System whose crust is dominated by sulfur and its compounds. Carbon and nitrogen are also very common in the universe but we don't see them or their compounds in Io's crust. Why not?


texts: Table of absolute magnitudes of stars by spectral type and photometric band

texts: I would like to find a table where I can, say, look for K5 dwarfs and read off that stars of that type have on average an absolute magnitude of X.XX in the infrared L band and Y.YY in the G band. I've looked everywhere and I can't find a compilation of this kind of information. Does any such thing exist?


texts: Is there going to be a really bad Solar Storm in 2014?

texts:
Here are some Articles I've found.

Is it true? If it is, How big could it be?, what might be possible or will be possible?, what will happen?


texts: Will we start seeing galaxies disappear due to Universe expansion?

texts:
Does the fact that universe is currently expanding at an accelerating rate means that far far away objects (FAO) might start disappearing with time, set aside the "red shift"?

I'm saying this because from what I understand until now:

we receive photons from FAO that were emitted long long ago
now due to space expansion, some of those objects are now moving away at speed greater than the speed of light, but at the time the photons we receive today were emitted, these FAO where not moving away faster than light

So there must be a point in time at which these far away objects disappear from view. How is this concept called?, or is it only a mistake I'm doing in my thinking?

Here's a simple drawing to illustrate:

at t=-11 billion years(By), photons are emitted in a slow expanding universe, and today we can view the object as it was
at t=-2 By, the object moves faster than speed of light, so photon emitted at that time will never reach us, even if we wait the 9 by + "expansion correction" needed...

I couldn't be convinced by the answer I found here, saying nothing disappears out of horizon. I don't see how it complies with that answer were event horizon approaches us.


texts:
I am new to this community and not quite sure if this is an appropriate question to be asked here. Anyways its just my curiosity. As far as I could know about WASP-18b is that, it orbits very close to its sun and takes less than 24 hours to complete one orbit. However this was not the case always, as the planet actually was formed in a cold part of the space and later got tug into of its sun's gravitational pull.

Given this is the case, I am curious to know how it was formed in the first place? If I am not wrong planets are formed when the debris orbiting in the gravitational pull of a star stick together overtime gets in to the form of the planetary bodies. So, if WASP-18b was not always so close to its sun and it was formed far away in cold space, then could there be a possibility that, this exoplanet was formed in an orbit of another star and overtime it got away from its original orbit and trapped in the tug of its current sun? (Its my imagination! just bare with it :))

Also could any one point me to good documentations from where I can learn more.

Thanks


texts: What is the frequency distribution for luminosity classes in the Milky Way Galaxy?

texts:
I'm working on a game concept that does some mild simulation of realistic stellar classes and luminosities. In particular, I'd like to roughly model the general frequencies of the classes and luminosities of the stars in the Milky Way.

Several sources, including Wikipedia's entry on stellar classifications, show a chart that includes the frequency distribution for spectral classification: the OBAFGKM categorization. So that's fine.

What I'm having trouble finding is any frequency distribution chart similar to that one but for the Yerkes luminosity categories: Ia+, Ia, Iab, Ib, II, III, IV, V, sub-dwarf and dwarf. I have a copy of the Hipparcos database, which contains a "Spectral Types" field, but it's highly incoherent text. Still, I could write some code to parse the values in that field to try to get a rough count of luminosity categories in those roughly 116,000 stars... but I'm a little perplexed that no such chart appears to exist already somewhere in Internetland. (Either that or my search-fu is weaker than usual.)

If anyone can point me to a chart of the frequency distribution for the luminosity categories noted above, or suggest a reasonably simple way for me to calculate those values myself, I'd appreciate it.

EDIT: Out of curiosity, I went ahead and did my own simple parsing of the spectrum fields from the Hipparcos dataset.

Out of 116472 rows, only 56284 (fewer than half) provided luminosity class data in the Spectrum field. Those 56284 rows broke down this way:

Ia0     16     0.03%
Ia     241     0.43%
Iab    191     0.34%
Ib     694     1.23%
I       17     0.03%
II    1627     2.89%
III  22026    39.13%
IV    6418    11.40%
V    24873    44.19%
VI      92     0.16%
VII     89     0.16%

Note: Around 1000+ rows gave an either/or value for luminosity class (e.g., "M1Ib/II"). In these cases, I counted only the first value provided. This probably skewed the results slightly compared to counting both luminosity classes.

I'm still very curious to know whether anyone else has produced or located a similar table of frequencies for the luminosity classes, if only to see how my very trivial analysis compares.


texts:
Say I'm standing up straight, and I draw a straight line from my core through the top of my head (perpendicular to the ground). What is the probability that that line intersects with a star?

EDIT: I'm not trying to exclude any stars. This should include stars which we've observed and stars which we haven't yet observed but can predict due to other things we've determined (like the overall star density of the universe). Also it should include all stars regardless of naked eye magnitude limit.


texts: Double Planet Orbiting Wide Binary Star?

texts:
Right now I'm writing a sci-fi story that develops in a double planet. But that's not all, the double planet (along with a couple more planets) orbit a big star and the whole system is orbited by another star (about 100 AU from the central star.)

I need to know how implausible this is. I don't want my story to drift from speculation into impossible worlds. I'm guessing that the occurrence of such a planetary arrangement is very low likely, but is such a system even possible at all?

Now, if it's possible, how messed up the days and seasons would be? I'm bending my brain to figure out what would be the length of day and night in such a system (the planets are very close to each other.)

Any help with this will be really appreciated.


texts:
Solar Flares obviously release extreme amounts of energy and extend thousands of miles out into space.

Because they are so big I would like to be able to observe some of these events through a telescope but I am wary of damaging both my eyes or the sensitive telescopic equipment.

How might I safely observe a solar flare through an amateur telescope? Might this be solved by using light filters to dim the amount of light entering the telescope or is a more specialist piece of equipment required?


texts: Are there galaxies with 2 or more super massive black holes orbiting each other?

texts: We now know that most stellar systems have 2 or more stars orbiting each other. Do we know of any galaxies which have 2 or more super massive black holes orbiting each other? Is it possible?


texts: Evidence that galaxies are made of billions of stars?

texts:
What is the evidence that galaxies are made of billions of stars?

Even faint galaxies?

Can spectral analysis distinguish galaxies from individual stars?


texts: Why do the upper-atmosphere clouds of Venus appear to have that V shape?

texts:
IF I understand correctly, the atmosphere moves in the same direction as the rotational spin, but about 60 times faster. It is driven from the hot side of Venus to the cold side (the difference being great due to the planet's slow rotation allowing time for it to heat up and cool down). Is my understanding correct?

But why does the visible flow in the cloud tops describe a sideways V shape, with the flow appearing to move diagonally from the equator toward the poles? Is that also because the equator is hotter than the poles, driving the flow in that hotter-to-colder direction?


texts: Do the stars in irregular galaxies orbit anything?

texts: Do the stars of irregular galaxies (such as the Magellanic clouds) orbit a more precise point or are they just, I don't know, flying around? If they orbit, how stable would such orbits be, considering the overlapping of many different stellar orbits without any main ecliptic? This post doesn't mention irregular galaxies.


texts:
Many Martian meteorites have been found on Earth. It also is possible for Earth rocks to have landed on Mars. So I guess it would also be possible for meteorites on Earth to come from Venus. Have any such meteorites been found? Would it indeed be possible to find Venusian meteorites on Earth?
Martian meteorites are recognizable because we know the composition of the atmosphere of Mars. Do we know the composition of the Venusian atmosphere well enough for this purpose?
If we found a Venusian meteorite, would we be able to recognize it as such?


texts: How big would the Earth look from the Moon?

texts: Imagine standing (somewhere) on the Moon's surface. How big would the Earth look from the Moon, compared to the image of the Moon we see from Earth? What would be the ratio between the diameters of each body? How would this ratio be calculated? Any help with answering such a calculation would be highly appreciated.


texts: Uranus' axis of rotation-when discovered?

texts: Who discovered it, and how was that accomplished.  I had assumed it was known before Voyager 2 arrived at the planet since it isn't mentioned in  JPL voyager 2 Uranus Approach


texts: Are the most distant known objects in the universe more than 14 billion light years away?

texts:
When I hear about the most distant objects in the universe, such as the recently discovered galaxy GN-z11, their distances are usually stated to be a little under 14 billion light years away....
But, the observable universe is 46.5 billion light years in radius... So..
Does the 13.4 billion light years or so distance for GN-z11 factor out the expansion of the universe itself?  So-called 'co-moving distance'?  Or is it actually, currently about 13.4 billion light years away?


texts: Could there be a brown dwarf in our solar system?

texts: Is it possible that a brown dwarf object could be orbiting our star and undetected so far by wise surveys?


texts: Is there a variation of ISO 8601 for julian calendar dates?

texts:
If I want to write a date in the Gregorian calendar, I can write it in the ISO 8601 format; today would be 2016-02-20. The advantage is that there are many parsers available to let computers read in this kind of date. But what if I want to write a Julian calendar date, as astronomers often do? Is there a similar widely-accepted format, and selection of parsers, to use with the Julian calendar.

While I'm at it, what if it's an ancient date where we don't know how many solar days ago it was; instead it is an astronomical phenomenon calculated in Terrestrial Time (more or less atomic time) or by dating decay of radioactive elements? Is there a standard way of writing such a date?


texts:
That actually sums up my question nicely: How did single dish (or single receiver) radio telescopes originally generate images? - or at least 2D intensity maps or contour plots.

Early radio telescopes were actually not much more than a Directional Antenna pointing up, connected to a sensitive receiver with various filters, maybe an IF stage or two, but no demodulation. You measured the "noise" signal strength as a function of time on a chart recorder - pen and ink.

I think there were radio images generated and published well before there were high granularity interferometers and computational correlators. The early ones were contour plots, on 2D plotters - again pen and ink.

How was this done? How did, say, a single dish antenna with a single feed generate radio maps?

edit: It doesn't necessarily have to be a dish antenna - it's the single receiver part I'm interested in.


texts:
I want to create a semi realistic star system generator but I am not sure where to "put in" gas giants. My guess is they can form pretty much anywhere within the sphere of influence of a star at the right circumstances.

I am still debating how to exactly go about generating the system but it won't be super complicated. I probably have plop in a star with a random mass and based on modifiers like distance, reflection, greenhouse and random chance plop in certain types of planets. It does not need to be realistic but it needs to feel realistic for none astronomy experts.


texts: Is the eccentricity of a planets orbit relevant to finding whether the distance between said planet and its host star determines seasons?

texts: The tilt of the Earth is the main factor that determines the seasons. It's also true that the eccentricity of Earth's orbit is very close to zero, implying that aphelion and perihelion distances are relatively close. If the eccentricity of Earth's orbit were close to but less than one, then would the distance between the Earth and the Sun become a dominant factor in determining the seasons?


texts: Why is it so difficult to discover the earth Trojan?

texts:
It is discovered only in 2011.

Why is it so late?

We know where to find it.


texts: Solution for counting atmospheric layers of planets according to relational data

texts:
This math states, that by multiplying the gravitational pull (we assume square of density of the planet), by its inclination (being how high and out it goes) and subtracting that loop from the velocity in a slope formula that measures to the borderline of being horizontal on the sun's x-axis, most parallel with the solar system's other planets, relegating the offset in order as to consign the preferred balance of the planet with the Sun. We can take then from the furthest point out once the Sun can lesser not take from the product and sum and difference of the clouds on the planet so that we may therefore continue on and calculate the priori aft dividing by 100 to delineate in a reciprocity to the true count, the atmospheric makeup of which the count of surface bars is.

This site is were to get the constants for the planetary facts you will need to use the formula.

https://nssdc.gsfc.nasa.gov/planetary/factsheet/

n = Orbital Velocity - root of density * inclination to sun
round((Apogee - n)/100) = Planet's Atmospheric layer count;

This is an easy formula to figure out how many layers of atmosphere your favorite planets have on them. It's preliminary because I don't have the satellite data from each planet to synthesize with my data from this formula.

If this is wrong, please tell me. If it is not, I suppose tell me. But I am only in search criticism and laud.

NASA Deg.   Real Deg.   Planet + Rank   Density Orbital Vel.    Apogee  Bars (Atmospheres)

17.2        15.3        Pluto+4         2095    4.7             7375.9  6.6 Bars

7           5.1         Mercury+3       5427    47.4            69.8    2.5 Bars

1.8         3.7         Neptune+2       1638    5.4             4545.7  44.01 Bars

1.3         3.2         Jupiter+1       1326    13.1            816.6   34.13 Bars

1.9         -           Mars -0         3933    24.1            2492.2  2.1 Bars

2.5         -0.6        Saturn -1       687     9.7             1514.5  15.07 Bars

3.4         -1.5        Venus -2        5243    35.0            108.9   2.52 Bars

0           -1.9        Earth -3        5514    29.8            152.1   3.22 Bars

0.8         -2.7        Uranus -4       1271    6.8             3003.6  31.06 bars


texts: How do/did we figure out that planets move in orbits?

texts:
I've learned that planets move in orbits around the Sun, but I really don't know how I would come to this conclusion myself. I've only seen planets in the sky a couple times (knowingly), and I am curious how we know for certain today that planets really move in orbits around the Sun (i.e., rather than moving but not around the Sun or moving around the Sun in a shape that is not a regular path).

I've also heard that people in the past knew about orbits even when they thought that Earth was at the center of the solar system. How did they figure this out in their times with their technology?


texts: What is the closest heavenly body to our solar system

texts: What is currently the closest known heavenly body, brown dwarf, rogue planet or other to our solar system? At one time it was Alpha Centauri.


texts: Why can't we observe a solar eclipse and the preceding/following lunar eclipse from the same place?

texts:
Lunar and solar eclipses go in tandem so the two often happens two weeks apart.

However I noticed if there is a solar eclipse in my area then the preceding/following lunar eclipse is not visible from my area because the Moon is set when it happens.

Similarly when lunar eclipse is visible in my area, the corresponding solar eclipse is not visible at my area because the Sun is set.

Recently the same thing happens in July 2 when there were a total solar eclipse on the other side of Earth, and soon we will see a partial lunar eclipse from my area.

The only exception was when the eclipse happens near the sunrise or sunset.
For example on May 31 in 2003 a partial eclipse could be seen from Hungary at the morning when the Sun rose already partially eclipsed just few minutes before the maximum occultation.

Two weeks before that there was a total lunar eclipse where we could only see the beginning of the eclipse as the Moon was already set when totality occurred.

These eclipse pairs seem to occur almost exactly at the same time of day.
Why?


texts: We know that black holes can collide and merge, but if they each have a singularity at the core, how can the two singularities merge and become one? Wouldn't the mere fact of a second singularity in range of another make it not a singularity?


texts: Can you see something active in the sky apart from satellites? Can there be amateur time-domain astronomy?

texts:
What I mean is an event unfolding that is viewable by naked eye or telescopes, and doesn't take comparing days of footage to see moving pixels. Apart from satellites, is there something that moves or happens so quickly that humans could sense it within a reasonable time? The whole sky is like a still painting, I'd love to see parts of it being "animated". Things on my mind are:

Crescent moon occulting a star (revealing or hiding it)
Solar and Lunar eclipses

Or more exotic things like:

Stars exploding (catching that as it happens, is it even possible?)
Other planets' moons moving a reasonabl/detectable amount in 30 minutes perhaps
Slow pulsar blinking (they are still super fast afaik)
Star changing its brightness and not blinking due to haze/turbulence


texts: What can a regular person use to view the stars?

texts: My favorite thing in this world is looking up at stars but I live in NYC so I can't ever see stars. I was wondering if anyone knew of a device I can attach to a smartphone that would let me look up at the stars and actually see some? I can't afford to buy an expensive telescope. I'm a student and I just want to see some stars. Thanks to anyone who can help.


texts: How can we be sure that we have identified very distant stars correctly?

texts:
From reading this question on calculating distance to stars and from a bit of background reading on the standard candle theory I still don't see how we can confirm that a star we see at one distance and a star we see at another distance (redshifted and at a different apparent luminosity) are the same type.

Beyond a certain distance, parallax is not possible, so how can we confirm star X and star Y are the same type therefore the difference in redshift and luminosity means Y must be at distance Z. How do we know it isn't a different type of star at a different distance?

Or are we just saying they look similar, so probably are?


texts: My question is if I put you down somewhere in the Milky Way galaxy in any given period of time, is there a way for you to tell when and where you are based on stars or constellations? Or some known astronomical event like a supernova? How accurate would that be?


texts: How close to Earth were the asteroids moving through this Hubble composite image?

texts:
Recently Hubble saw several asteroids traveling erratically through a deep field perspective.

What region of the sky is this, and how near were these asteroids to the Earth?

The photo is composed of multiple shots and the asteroids have copies because of Hubble's movement through frames as the photo assemblage shifted.

Was Hubble viewing towards the plane of the solar system and the asteroid belt?
How did they explain that there were five in the same narrow frame?

https://astronomynow.com/2017/11/03/hubble-sees-nearby-asteroids-photobombing-distant-galaxies/


texts:
Is there the possibility for a habitable Planet around a red dwarf? It is not required that the whole surface is habitable, but it shouldn't be a tiny spot.

After reading a bit I assume the planet would be tidally locked to the star, with the side that faces the sun looking like an archery-target (from center to border: scorching, hot, warm, cold, freezing).

Also from space it would look like a Rugby Ball (exaggerated) pointed at the star.

But would such a planet be stable enough to develop and support life more complex than the simplest forms?

I do not need a specific star, but the general answer if this is possible, however unlikely (though a value how unlikely would be nice).


texts: What if the atmosphere of Titan were like the one on Venus?

texts:
The current mean temperature in Titan is −180 ºC (93 K), the same as in Saturn.

But I was wondering what would be the case if the atmosphere of Titan would be like the Venus atmosphere, whose composition is mainly CO2 and has a pressure of 90 bar, instead of approximately 1.5 bar.

What temperature could Titan reach in that case? Would the greenhouse effect allow the moon to be heated enough to maintain liquid water or is it too far from the Sun to be affected by any kind of greenhouse effect? Maybe the latest question allows to ask another one: How far from the Sun could Venus be so it can have temperatures above 0 ºC?

The question may be answered given two possible alternatives:

Same composition as Venus, but same pressure as in Titan.
Same composition and pressure as in Venus.


texts: What is a debris field called before it becomes a planetary ring?

texts: As a body of a suitable size and mass moves within the Roche limit of a planet, it will disintegrate. This will form a debris field that will eventually form a planetary ring. Is there a name given to this debris field before it forms a ring? Something like a Roche train or Roche field seem appropriate but I can't find any reference to such


texts: Why can we see stars but not astronaut on the moon

texts:
A question came up today in astronomy class and it got me wondering all night.

How is it that if stars are so far away, we can see them so clearly but we cannot see astronauts on the moon.


texts:
If a spaceship was to travel to the barycenter of a binary star (the stars having identical size and mass) and come to a stop there, would the people on board this spaceship find that time is moving slower there (relative to the passage of time experienced outside of the gravitational fields of this binary star) due to the two stars overlapping gravitational fields at this point in space?

Does time dilation increase within overlapping gravitational fields?


texts: How many planets does Omicron Persei have?

texts:
Omicron Persei is a binary star. I did not find information on how many planets it has and if we can actually determine it currently. However, I do know that most stars have planets orbiting around them and so I assume there are such planets.

For those who are interested, Futurama featured the planet Omicron Persei 8 - the 8th planet in that system. I was curious as to if there is an 8th planet.


texts: Why did Venus not lose its atmosphere without a magnetic field?

texts: It is often stated that the magnetosphere not only shields the planet from cosmic radiation, but also prevents atmospheric loss. Why then did Venus not lose most of its atmosphere if it doesn't have a strong magnetic field? Is there another mechanism at play, or is the statement about the importance of magnetosphere to atmospheric loss prevention wrong?


texts: Constant interval between pulses from all pulsars

texts:
It was found from the observations of the radiopulsars that the intervals between their pulses are changing
periodically, besides, for all pulsars this period is the same and equal to one stellar (sidereal) year. Explain
this phenomenon


texts: Why a mass transfer in a binary star system, from the less dense star to the denser star forms an accretion disk? Why the material does not fall directly to the denserstar?


texts: Does the astrology of constellations relate to its position during the year?

texts: For example, pisces birthdays are from Febuary 19th-March 20th. Would the sun be near or at the pisces constellation during Mid febuary to mid march? Basically, I was wondering whether the astrology we use now is related to the sun's position relative to the zodaic constellations.


texts:
How long ago the Jezero crater was formed on Mars?
And is it a meteorite impact  crater?


texts: Is it possible for a natural moon to have a natural moon? What is the limit to when satellites on satellites becomes to unnatural for it to be possible?


texts:
Could water from Mars be fresh enough to drink? Could one run the water through a home water filter? I understand when water is frozen the salt is ejected as brine so would newly formed snow or ice be drinkable?

https://earthscience.stackexchange.com/questions/16076/how-salty-can-snow-sleet-be


texts: Precautions when retrieving material originated from space

texts: I'm assuming that when retrieving a meteor for study and later display, there are precautions taken against possible radiation coming from it. Is my assumption correct and are there other precautions taken such as protection from biohazards? (e.g. is it not possible for a meteor to carry a foreign viral agent?)


texts: Why are ice giants not considered suitable for life?

texts:
Ice giants are currently not considered suitable for life due to temperature, pressure and composition.  This paper gives some parameters for life in regards to temperature, pressure and composition:

Temperature: $-15\ ^\mathrm{o}C$ to $122\ ^\mathrm{o}C$
Pressure Upper Limit: $1,100\ \mathrm{atm}$, and
Composition to Include: liquid water, $C$, $N$, $S$, and $P$.

I'm trying to find out what the conditions of the hot dense ammonia/water mantle border with the hydrogen/helium atmosphere are, in regards to temperature and pressure. We know the mantles are under very high pressures, have liquid water (though in a likely different state due to pressures), C, N and S. What are the pressures and temperatures at this mantle border?  Specifically in reference to Uranus and Neptune.


texts:
Why do telescopes use mirrors that simply reflect photons, when they instead could be covered with large sensors to register them? Reflection is all good and well, all thanks to silver and beryllium for that. But wouldn't it be better to electronically register the photons directly instead of after having them bounce around between stupid mirrors? Would any data be lost in a pure CCD-telescope without any mirrors or lenses?

Couldn't a large wired CCD light sensor send the detected signals further in a smarter way than a stupid physically reflecting surface can do? It's the same photons and the telescope itself doesn't generate any new information about the distant galaxies it reflects. Why physically bend mirrors for adaptive optics, instead of bending the raw binary data with an algorithm for the same effect?


texts:
I read about Hawking and Leonard Susskind and their debates about whether information is destroyed or not if it falls into a black hole.  It was resolved in favor of information always being conserved, preserved, whatever (i.e. not destroyed).

I see how this could be consistent with determinism.  However, how is this in keeping with quantum physics, when there is so much randomness.  It seems that at any moment, all matter, energy, even every atom in our bodies, is indeterminate and "scrambled" somewhat.  Even if you could pinpoint or describe every atom and its position perfectly in your body at one moment, by the next moment there is uncertainty.  So how is this compatible with conservation of information?


texts: How was the time of equinox measured in ancient times?

texts:
In the book "The Crime of Claudius Ptolemy" by Robert Newton, on page 81, it reads as follows:

  When we measure the time of a solstice, we measure the time when the
  meridian elevation angle of the sun has an extreme value, a maximum in
  the summer, and a minimum in the winter.... When we measure the time
  of an equinox, on the other hand, we measure the time when the
  meridian elevation takes on a specified value.

What does he mean by a "specified value". What specified value?

Also, how would the ancients have determined the time of the maximum, or the specified value? Time devices in ancient Greece, like water clocks, I suppose would have significant errors. Would they just start the water clock when the sun rose, then marked its level when the "specified value" occurred? That method would seem to have a lot of error.


texts: Is there another habitable planet in our galaxy?

texts: I have wondered, what'll happen when we destroy our planet or it becomes uninhabitable for human life? Is there another option than the complete extinction of the human race? Can anyone give me a logical response?


texts:
In the past mornings I have used to get out before sunrise for a stroll, to get some exercise while not meeting other people.

I live in Eindhoven (NL), and I get out of home at around 5:30 AM, when the Sun is not yet risen, the sky is just about to start getting brighter.

At about 30-40 degrees to the right from the rising Sun point, about 20 degrees above the horizon, I see a bright white star.

About 5 degrees to the left of this star, I see two reddish objects, about 2 degree apart from each other, much more faint than the other star.

Which stars are they? I have tried to find them in a sky map, but haven't been able to find anything resembling them.